The present invention relates to a method of providing access to an IP multimedia subsystem for a subscriber communicating in an access communication network.
Mobile communication network GSM were designed on the basis of switched circuit (CS) connection. The advent of GPRS (General Packet Radio Service) introduced packet switched (PS) connection from mobile terminals and network servers. However, the services offered to the user were still determined by the network operator.
The third generation partnership project (3GPP) has defined a new standardized architecture known as Internet Protocol (IP) Multimedia Subsystem or IMS for offering multimedia services in a packet switched based domain to end users whereby the network operator accessing to IMS services controls the services offered by the IMS.
IMS is access agnostic to the extent of the network requesting access to its services so that in a multi access environment it ensures services availability to all access networks, subject to the limitations of the access network or the specific user requesting access to IMS.
The core network of an IMS architecture comprises database elements and IMS control elements.
Home subscriber server (HSS) is the main database element of IMS and is responsible for holding user-related data such as user identification, numbering, addressing, security, location management and user profile information.
IMS control elements are call session control function (CSCF) servers which comprise proxy-CSCF (P-CSCF) which is the first point of contact for users within the IMS and is responsible for forwarding messages, particularly session initial protocol (SIP) messages, from the user to the other CSCF elements, serving-CSCF (S-CSCF) which performs registration, session control for registered end points, interaction with service platforms/application servers of IMS and interfacing to HSS for users registered therein, and interrogating-CSCF (I-CSCF) which is the contact point for external networks and is responsible for finding the appropriate S-CSCF from HSS.
In IMS roaming the packet network resources of the visited mobile telecommunication network in which a user in registered are used to connect the user to IMS core network, which may reside in the home mobile network (HPLMN) or in the visited mobile network (VPLMN). In any case, all the core elements of the IMS are always located at the home network and the subscriber is always registered with the S-CSCF of the home network for the purposes of charging and interfacing to the HSS.
When an IMS subscriber is located at his home GPRS/IMS network, he accesses IMS services through a home network access point, such as the gateway GPRS support node (GGSN).
A roaming scenario supported by IMS is the so called Home GGSN (HGGSN) model where the roaming subscriber uses the home network access point HGGSN to access IMS services. In this scenario, the subscriber is located under a visited network while user plane and control plane data are sent via the HGGSN located in the subscriber's home network.
This is a typical GPRS roaming situation where the access point HGGSN for accessing IMS services is in the home network and a GPRS tunnelling protocol (GTP) is used across the inter-operator backbone network of the home and visited network.
One of the major business requirements for an operator is to apply a proper tariff to the user that are in roaming condition, both outgoing roamers as well as the incoming roamers. In particular the requirement is to identify whether a user is in roaming condition or not and eventually in which network the user is actually roaming. This requirement is not specifically addressed by 3GPP or any other standards.
A workaround would be possible by correlating HGGSN generated charging information and IMS generated charging information. However, this would eventually work for offline charging but it does not work for online charging since no OCSs (Online Charging Systems) have the ability to perform the correlation process in real-time.